Electric valve converting system



May 27, 1941. E. E. MOYER EIAL ELECTRIC VALVE CONVERTING SYSTEM Filed Sept. 16, 1939 In ventors. Etmo E Moyer,

Lysle W Morton August SchmidtJr.

Their Attorney.

Patented May 27,194!

UNITED STATES PATENT OFFICE 2,243,571 morale vaLvn convnn'nmo sYs'rsM Elmo E. Moyer, Scotia. Lyslc W. Morton, Schenectady, and August Schmidt, In, Niskayuna, N. Y.;

ration of New York asaignors to General Electric Colum a o 'l fl- Application September 16, 1939, Serial No. 295,342

. foregoing difliculties are obviated by means of an 13 Claims.

'main supply transformer has tended to cause unequal distribution-of the load between the secondary windings especially if the secondary windings are interconnected by an interphase trans former, one of the secondaries being subjected to arrangement for continuously indicating the currents flowing from the secondary transformer windings and controlling theelectric discharge the larger load when the phase rotation of the main transformer is in one direction and the other secondary having the larger load when the phase rotation of the main transformer is in the other direction. In other cases, the cause of the unbalance has been traced to differences in the voltage drop of the arc discharge paths associated respectively with the transformer secondary windings. The varying grid characteristics of the electric discharge valves also are a cause of unbalance. Other causes, more or less obuure, have been found to contribute to the unbalance.

Various schemes have been suggested to overcome the tendency to unbalance in the abovementioned power systems including the provision of reactance in the connections associated with that one only of the secondary windings of the main supply transformer. the output of which tended to rise above the output of the other secondary windings. Various other schemeshave been employed where an existing de ree of unbalance is compensated foror prevented by some valves in response to any unbalance indicated, thereby to restore balanced conditions or better still to prevent imbalanced conditions.

It is an object of our invention, therefore, to provide an improved electric valve converting apparatus which will overcome the above-mentloned disadvantages of the arrangements of the prior art and which will be simple and reliable in o tion.

It is another object of our invention to provide a new and improved electric valve converting apparatus.

A further object of our invention is to provide an electric valve converting apparatus wherein means are provided to prevent unbalance between the currents flowing from the plurality of secondary windings of the transformer of the electric valve converting apparatus.

A still further object of our invention is to provide a current balancing arrangement for an electric valve converting apparatus wherein the balancing arrangement does not compound with current but rather the force tending to restore the balance always tends to be suppressed toward zero.

It is another object of our invention to provide an electric valve converting apparatus wherdn the currents flowing tom the plurality of secondary windings of the electric valve converting apparatus transformer are compared and any initial adjustment. These methods of preventing or compensating for the unbalance have been found to be inadequate in certain cases due to the random shifting of the unbalance from one of the transformer secondaries to the other during' the operation. This shifting is due apparently, among other causes, to change in transformer temperature, the balance being impaired after a period of operation on heavy overload. In other cases, the shift is apparently caused by changes in voltage drop through the arc discharge apparatus with age and temperature under which conditions the unbalance between the transformer secondaries tends to be reversed.

In accordance with the present invention the unbalance between these currents is used to control the grid excitation of the electric discharge valves in such a manner as to restore balanced conditions.

- In accordance with the illustrated embodiment of our invention we provide an electric valve converting apparatus including a plurality of electric valves and a transformer having a pair of Y-connected secondary windings interconnected. by an interphase transformer. In order to balance the currents between the two Y-connected secondary windings current transformers I rent bias potential is applied to the control electrodes of the electric discharge valves associated with the respective Y-connected' secondary windings. Thisbiaspotentialissuchastocause member type.

a decrease in the load carried by the secondary transformer winding and associated discharge valves carrying more than their share of load and simultaneously to cause an increase inithe load carried by the secondary transformer winding and associated discharge valves carrying less than theirshare of load. Switching means are also provided for cutting out the operation of the balancing means on the electric valve converting apparatus, whenever this is desired.

The novel features which we believe to be characteristic of our invention are set forth with P81? ticularity in the appended claims. Our invention itself, however, will best be understood by reference to the following description taken in connection with the accompanying drawing in which the single figure thereof diagrammatically represents an embodiment of ourinvention.

Referring now to the single figure of the draw-- inc, we have illustrated therein a system embodying our invention for transmitting energy from a polyphase alternating current supply circuit to, having three phases, to a direct current load circuit it. An electricvaive translating appara tus including the main electric discharge valves ii! to H, inclusive, and a transformer I8 are arranged to transmit the energy between the supply and load circuits ill and ii, respectively. The transiormer is may comprise a primary winfiim I9 and groups of Y-connected secondary windings 2D- and 2i, the neutrals of which are intercom nected by means of an interphase-transiormer 22 to produce a form of sir-phase system of the double (type The midpoint 23 of interphase transformer 22 is connected to one side of the direct current circuit H, while the other side of the direct current circuit is connected to the cathodes or the main electric discharge valves ii to H. As indicated, suitable fuses 24 may be connected in series relation with the electric valves 12 to H, inclusive. The electric valves l2 to H are preferably of the type employing an ionizable medium such as a gas or a vapor, and each comprises an anode 25, a cathode of the colt-reconstructing type such as a mercury pool cathode 26, and a control memberv 21 of the make-alive or the immersion ignitor control The immersion ignitor control members 2'! are of a material having a substantially greater electrical resistivity than that of the associated mercury pool cathode 26. These control members may be constructed of a-suitablo material such as boron-carbide and silicon-carbide, or of the material disclosed and claimed in U. S. Letters Patent No. 1,822,742, granted September 8, 1931, on an application of Karl B. McEachron,- and assigned to the assignee of the present application. In order to establish an arc discharge between the anode 25 and the cathode 28, it is necessary that a predetermined minimum critical flow of current be transmitted through the immersion ignitor control member 21. The main or power are discharge path of each of the electric valves 12 to H, of course, lies between the anode 25 and the cathode 26.

Excitation circuits 28 to 33 are associated with electric valves If to II, respectively, and may be connected to be responsive to the anodecathode voltage of the associated main power electric valve. Each of the excitation circuits 28 to 33 includes 'a control electric valve 34 which may be connected between the anode 25 and the immersion ignitor control member 21 of the associated electric valve through a current limiting means such as a fuse 35. a resistance 36 75 and an inductive reactance 31. The fuses protect the control electric valves 34 while the resistor 38 limits the surge peel; current which might flow through the auxiliary control electric valve in the event the main electric discharge valve does not conduct. The inductive reactance 31 combined with capacitor 38 which is connected in parallel with the control electric valves 34 decrease the high frequency oscillations which are undesirable from the standpoint of radio interference. The control electric valves N are preferably of the type employing an ionizable medium and each comprises an anode 39, a cathode 40, a filament or cathode heating element 4i therefor, and a control member or grid 42. As described the electric valves 34 may be of the type employing a screen or shield grid 43 which is connected to a point less positive than the anode and may be connected directly to the oathode. A capacitance M is connected between the cathode 4i! and the control grid 42 to absorb transient voltage variations which may be presout in the grid circuit due to the anode circuit. A current limiting resistance 45 may be connected in series relation with the control grid it and the said or excitation circuit described hereinafter. In order to impress on the control grlds -52 of electric control valves 3 in excitation circuits 28-33 alternating voltages preferably having a lagging phase displacement relative to the anode-cathode voltages of the re spective control electric valves, we employ a transformer 45 having a primary winding 47 and groups of secondary windings 58 and 9. The primary winding 4'! of transformer 46 may be energized with alternating potential from any suitable supply which has a fixed phase relationship with said anode supply, and is illustrated as being energized from alternating current supply circuit it. The secondary windings 4t and as of grid transformer 68 are connected in zigzag-Y relationship so that the alternating grid potentials have a lagging hase displacementrelative to the potentials of the-secondary windings 20' and 2| of the main power transformer M. This lagging phase displacement should preferably be in the neighborhood 015190 electrical degrees whereby the bias shift method of grid control may suitably be employed as described and claimed in U. S. Letters Patent No.

155L987, granted January 3, 1928, upon an application of Albert H. Mittag. By impressing a direct current bias upon the neutrals of the sec ondary windings 48 and 49 of grid transformer 41, a shift in the phase of the grid potentials of control electric valves 34 relative to the anode cathode voltage of these valves occurs and in this manner the valves may be caused to conduct for longer or shorter intervals of time de-. pending upon whether the grid excitation is advanced or retarded. lit-will be understood by those skilled in the art that in the arrangement described above a positive direct current bias impressed. upon the neutrals of the secondary windings 48 and 49 of grid transformer 48 will advance the phase of the grid excitation of control electric valves 34 and cause them to be conductive for a longer interval during each cycle. Conversely a negative bias will retard the phase excitation and render control electric valves 34 conductive for a shorter period during each cycle. In order to obtain this direct current bias to advance or retard the grid excitation of control electric valves 34 we provide a potem tiometer regulator 50 the moving arm ll of which The other terminal of potentiometer regulator.

50 is connected tothe cathodes of control electricvalves 34 through cathodes 26 and immersion ignitor type of control electrodes 21 of main electric discharge valves I! to II. By providing two secondary grid transformer windings 48 and 49 each associated with a separate group of three electric discharge valves it is poss ble to impress a different direct current bias potential upon the neutral of secondary winding 48 than upon the neutral oi secondary winding 49 so that control electric valves 34 and associated main valves l2, l3 and I4 may be made to carry more or less current than controlvalves 34 and associated main valves l5, l6 and H as will be described in detail hereinafter. However, the direct current bias potential obtained from poten-' tiometer regulator 50 impressed upon terminal 54 will through control valves 34 effect all of the main electric discharge valves I! to II in the same manner. Itwill of .course be understood by those skilled in the art that potentiometer regulator 50 can readily be adapted to automatic'operation. For example, the moving arm 5! may be controlled in response to the voltage of the direct current circuit il whereby voltage regulation of this direct current circuit may be obtained.

Although we have described the electric valve converting apparatus as transmitting energy from a three-phase alternating current supply circuit II to a direct current circuit II, it will be understood by those skilled in the art that supply circuit l0 may comprise any number of phases and furthermore that energy might equally well be transmitted from direct current circuit H to alternating current circuit II or even between two alternating current circuits of like or different frequency.

Since actual operation has shown a persistent tendency toward unbalance between the main power currents. flowing from the respective secondary transformer windings to the electric discharge valves of electric valve converting apparatus of the type described, we provide, in accordance with our invention, an automatic current balance control by which to maintain a balance between thecurrents flowing from the re to the cathode bus for the purpose of minimiz ing any capacity effect between the secondary coils and the anode conductor or the core structure. The secondary outputs of current transformers 56, 51 and 58 which are associated with the anode supply lines leading from Y-connected elements of thermionic electric valves 62, 63

and 64. Each of these thermionic valves is provided with a pair of anodes 65 and 66. Currents flowing from current transformers 5G to 58, respectively, are connected to anodes 65 of thermionic valves 62 to 64 and currents flowing from current transformers 59 to 6| are connected-to anodes 66 of thermionic valves 62 to 64. These rectifiers 62 to 64 are each provided with a cathode 61 of the hot cathode type, the filament of which is suitably energized for heating purposes from the filament transformer 68, the primary winding of which is supplied from a suitable source of alternating current. The direct current output of current transformers 56 to 58 when rectified by electric valves 62 to 64 is filtered or smoothed by means of a reactor 69 and a capacitor 10 so that not the peak currents but rather the average currents are metered. A potentiometer H is connected across capacitor Ill so that the direct current potential appear ing across potentiometer 1| is to all practical purposes proportional to the average anode current flowing through the three anodes associated with the Y-connected secondary winding 20 of transformer 18. Similarly, the output from current transformers 59, and Si when rectifled by thermionic valves '62 to 64, is filtered by reactor 12 and capacitor 13. The potentiometer spective secondary windings of the main power transformer. In the illustrated embodiment we have provided a current balance control by which a balance is continuously maintained between 'the currents flowing from the two Y-connected secondary windings I0 and II of main power transformer l8. Accordingly, we have provided a plurality of current transformers 56 to SI arranged so that each has its'primary winding connected in series with one of the main anode power supply lines associated with electric valves- I! to 17, respectively. These current transformers are used to indicate the currents flowing through the respective anodes of electric discharge valves I! to 41v and are provided with I4 is connected in parallel with capacitor 13 and the potential appearing across this potentiometer is proportional to the average. anode current flowing through the anodes of electric discharge valves ii to I] associated with Y-connected secondary winding ll of transformer III.

The polarities of the current transformer windings as indicated by the arrowheads associated with current transformer 56, are such that when current flows through the primary conductors in a direction from anode to cathode of main electric valves I! to II, respectively, a secondary current flows from anode to cathode of rectifier valves 2 to 84. Because of the fact that an excessively high inverse voltage appears. across the secondary windings of the current transformers 56 to I, respectively, at the time when the main anode currents of electric discharge valves I! to I! cease to flow, we have provided electric discharge valves I5 connected across the secondary winding of each of the current transformers 56 to 6| so as to suppress the inverse peakand also to eliminate the high frequency oscillations which would otherwise appear across the secondary windings of the current transformers. T5 are connected so as to become conductive in a reverse direction with respect to valves 62, 63 and 64. It will be understood by those skilled in the art that valves 15 could be replaced by means of large resistors, although in this latter case a certain amount of current would continually flow through the resistors and the necessity of call- These electric discharge valves bration oi the arrangement would occur in the interest of more accurate metering of the main anode currents.

In order to compare the direct current potentials proportional to the average currents flowing from secondary transformer windings 20 and 2| potentiometers H and 14 have a common positive terminal 16 and isolated negative terminals 11 and 18, respectively. If the currents flowing from each Y-connected secondary winding 28 and 2| are equal then the potentials appearing across potentiometers H and ll, assuming the constants of the circuit to be identical, are equal and no potential difierence should exist between terminals ii and 18. If, however, the currents flowing through electric discharge valves l2, H I

and H are greater than the currents flowing through electric discharge valves i5, i6 and i1, then the potential difference between terminals 16 and I1 will be greater than the potential difference between terminals 16 and '18 and terminal ll will be at a negative potential relative to terminal 18. Conversely, if the currents flowing through electric discharge valves l2, l3 and H are less than those flowing through electric discharge valves l5, l6 and ll then the potential diiferenoe between terminals it and TI will be less than the potential difference between terminals l6 and 18 and the potential of terminal '18 will be negative with respect to the potential of terminal 11. Potentiometers II and 14 are provided with adjustable sliders l9 and 3E), respectively, for purposes of adjustment which will be described in detail hereinafter and the potential diiference between terminal 16 and slider 18 is used instead of the potential difference between terminals 16 and TI. Similarly, the p0- tential difference between terminal 16 and slider through main electric discharge valves l5, l6 and I! there will be no potential difference between sliders I9 and 80 assuming that they are initially placed in the same position on potentiometers H and I4, respectively, and similarly, terminals 8| and 82 of resistances 52 and 53, respectively, will be at the same potential. The grid excitation circuits of all of the control electric valves 34 therefore will have the some direct current bias potential impressed'upon them by virtue of potentiometer regulator acting through terminal 54 intermediate resistances 52 and 53. 11!, however, more current flows through main electric discharge valves I2, l3 and I4 than through electric discharge valves l5, l8 and I! a negative direct current potential will appear at terminal and 2|. Thus it is observed that if unbalanced currents exist, equal and opposite bias potentials are applied to the respective excitation circuits in order to cause some electric discharge valves to carry more current while others are caused to carry less current so as to restore balanced conditions.

' In order that any one of the rectifier valves 62, 63 or 84 may be replaced in case it should become defective and without affecting the operation of the current balancing means to any great extent we have provided that the secondary windings oi the current transformers associated with diametrically opposite electric discharge valves, insofar as the firing period is concerned, both feed into the same two-element rectifier. For example, the secondary winding of current transformer 56 associated with main electric discharge valve i2 is connected to the anode of rectifier valve 62 while current transformer secondary winding 6! associated with main electric discharge valve ll is connected to anode 66 of the same rectifier valve 62. The same relationship is true with respect to the remaining current transformer secondary windings and rectifiers 63 and 64. By this arrangement it is possible -to remove thermionic rectifier G2, 63 or 64 from the circuit without greatly afiecting the operation as far as balancing the currents flowing from the two Y-connectedj seconds, windings 20; and 2l is concerned other than to change the sensitivity. It has been found that even with only a single one of the thermionic rectifiers 62, 53 or 84 connected in the circuit a considerable balancing effect is obtained which is improved when two rectifier valves are used and of course is much better when all of the currents flowing from the secondary windings 20 and 2! of transformer it are used.

In order that the electric valve converting apparatus may be operated without the current balancing means we provide double pole, double throw switches 84, 85 and 86, preferably of the make-before-break type, which are connected in series with the anode leads of the thermionic rectiflers 62, i3 and 64, respectively. Asingle pole switch 81 is also provided in order to short circuit terminals SI and 82 of resistors 52 and 53 and place them at the same potential when the current balancing means is cut out of the circuit. when switches 84, 85 and 86 are moved to the right the secondary windings of current transformers 56 to SI are short circuited and the current. balancing means is cut out of operation. These switches have been provided not only that the electric valve converting apparatus may be operated without the current balancing means but also to facilitate the replacement of the thermionic rectiilers 62, G3 or 84 without appneciably affecting other than the sensitivity of thecurrent balancing means in case one of these rectiflers should become defective.

ill of resistance 52 relative to the direct current The operation of the embodiment of our inventlon diagrammatically shown in the single figure of the accompanying drawing will be understood by those skilled in the art in view of the detailed description given above but will be briefly described by considering first the electric valve converting apparatus when operating with balanced currents flowing from the secondary windings 20 and 2| of main power transformer is. The main electric discharge valves l2 to I"! are rendered conductive in the proper sequence due to the action of the control'electric valves 34 and the grid excitation circuits 2Ito 33 associated therewith. The output of the electric valve converting apparatus may be controlled by varying the position of movable arm ii of potentiometer regulator 50 thereby varying the direct current bias applied to control electric valves l4 and either advancing or retarding the grid excitation thereby increasing or decreasing the power output. If the average currents flowing from secondary transformer winding 20 are equal to the average currents flowing from secondary transformer winding ii there will be no potential difl'erence between sliders I9 and 80 of potentiometers II and I4, respectively, associated with current balancing means 55 and hence no change I in the operation of the electric discharge valves will occur, assuming that the sliders l9 and 80 are in the same relative positions on their respective potentiometers.v If, however, the average currents flowing from secondary transformer winding 20 are gerater than the average currents flowing from secondary transformer winding 2|, a positive bias potential will be impressed on the neutral of secondary grid transformer winding 49 while an equal and opposite bias potential is impressed on the neutral of grid transformer secondary winding 48, whereby main electric discharge valves l2, l3 and ll will be caused to carry less current and main electric discharge valves I5, l6 and I! will be caused to carry more current thereby restoring the balance. It will be understood by those skilledin the art that a balance, the currents flowing will u ver become balanced. For this reason sliders I9 and 80 are initially adjusted relative to one another so that the apparatus will have a slight initial correction. Instead of sliders l9 and BI it will be understood by those skilled in the art that this initial correction could be produced by means of a flxed bias ofproper magnitude and direction. It is desirable that the potentials obtained from potentiometers II and II are as large as possible in order to minimize the degree of unbalance that must be tolerated. The heavier the load carried by the electric valve converting apparatus the better will be the operation of the current balancing means due to the fact that a larger direct current bias potential will be available at terminals ll and If for causing a-phase shift in the grid excitation relative to the anode potential of control electric valves 3|.

Although we have shown and described a particular embodiment of our invention as applied to an electric valve converting apparatus embodying a main transformer including a pair of Yconnected secondary windings interconnected by an interphase transformer, it will be understood by thosefskilled in the art that our current balancing means may equally well be embodied in connection with many other types of electric valve converting apparatus.

fall within the true spirit and scope of our invention. I

What we claim as new and desire to secure by Letters Patent in the United States is:

'1. In combination, an alternating current supply circuit, a direct current load circuit, a plurality of electric discharge valves interconnecting said circuits, a control electrode and an excitation circuit therefor for each of said electric discharge valves, means for obtaining a measure of the current flowing through said electric discharge valves comprising a control circuit including energy storage means for producing a direct current bias potential proportional to the difference between the average currents flowing through said valves, said bias potential being impressed upon said excitation circuits of said elec tric discharge valves to control the operation thereof in such a manner as to prevent any substantial unbalance between the currents flowing through said valves.

2. In combination a supply circuit, a load circuit, electric translating apparatus connected between said circuits and including a transformer and a plurality of electric discharge valves, said transformer including a primary winding and a plurality of secondary windings, a control circuit for preventing unbalance between the currents flowing from said secondary transformer windings comprising means including a plurality of current transformers for obtaining a measure of the current flowing from said transformer secondary windings, and means for controlling the operation of said electric discharge valves in response to any tendency toward unbalance indicated by said measured currents.

3. In an electric valve converting apparatus an alternating current supply circuit, a direct current load circuit, electric translating apparatus connected between said circuits and including a power transformer and a plurality of electric dis charge valves, said transformer being provided with a primary winding and a plurality of Y- connected secondary windings, an excitation cirpotentials proportional to the average currents flowing from said Y-connected secondary windings, and means for impressing any difference While we-have described what we at present I between said direct current potentials on the excitation circuits of the respective discharge valves associated with said Y-connected windings for restoring a balance between the currents flowing therefrom.

4. In combination a supply circuit, a load circuit, electric translating apparatus connected between said circuits and including a plurality of electric discharge valves, means for balancing the currents flowingbetween said valves, said last mentioned means including a plurality of current transformers and associated rectiflers for obtaining an indication of the currents flow ing through saidvalves, and means for modifying the operation of said valves in response to unbalance between said currents.

5. In combination a supply circuit, a load circuit, electric translating apparatus connected between said circuits and including a plurality of electric discharge valves arranged in groups, means for balancing the currents flowing between said groups of valves, said last mentioned means including means for obtaining an indica an alternating current circuit, a direct current circuit, means including a plurality of groups of electric discharge valves interconnecting said circults, a control circuit 101' balancingthe currents flowing through said groups of valves comprising means including a plurality of current transrcrrners for obtaining an indication of the currents transmitted by each of said of.

valves, and means responsive to said indicated currents for controlling the operation of said groups of valves to eliminate current unbalance between said groups of valves.

7. In combination, an alternating current supply circuit, a direct current load circuit, electric translating apparatus connected therebetween and including groups of electric valve means each having an anode, a cathode and a control 'member, an excitation circuit for energizing said control member, means including a plurality of current transformers for obtaining an indication of the average current flowing in the anode circuits of certain of said electric discharge valves, and means responsive to any unbalance between the current flowing in certain of said valves for impressing a potential on the excitation circuit of certain of said valves to restore balanced conditions.

8. In an electric valve converting apparatus an alternating current supply circuit, a direct cur- 7 rent load circuit, electric translating apparatus connected between said circuits and including a power transformer and a plurality of electric discharge valves, said transformer being provided with a primary winding and a pair of secondary windings each of said secondary windings being associated with a group of electric discharge valves, an excitation circuit for said plurality of electric discharge valves including an excitation transformer having a pair of secondary windings one of which is associated with each of said roups of electric discharge valves, means for maintaining a. balance between the currents flowing from said pair of secondary windings of said power transformer comprising capacitance means, means for charging said capacitance means in accordance with the current conducted by the valves associated with both of said secondary windings of said power transformer, and means for imprasing the voltage of said capacitance means on the secondary windings of said excitation transformer to reduce the conductivity of the valves associated with the transformer winding 08 711 8 more than its shart of the current and to increase the conductivity of the valves associated with the transformer winding carrying less than its share of the load .in order'to restore balanced current conditions between said groups of electric discharge valves.

9. In an electric valve converting apparatus an alternating current supply circuit, a direct current load. circuit, electric translating apparatus connected between said circuits and including a power transformer and a plurality of electric discharge valves, said. transformer being provlded with a primary winding and a plurality of secondary windings, said last mentioned means comprising a plurality of current transformers and associated rectifier-s for producing direct current potentials proportional to the average currents flowing from each of said secondary windings, means for impressing any dilference between said direct current potentials on the excitation circuits of the respective discharge valves associated with said secondary windings in such a manner as to restore balanced conditions between-the currents flowing therefrom, and switching means for cutting out the operaticn of said bsiancing means.

iii. In an eiectric valve converting apparatus an alternating current supply circuit, a direct current load circuit, electric translating apparatus connected between said circuits and ineluding a main power transformer and a plurality of electric discharge valves, said transformer being provided with a primary winding and a pair of secondary windings, an excitation circuit for said electric discharge valves including a grid transformer having a pair of secondary windings provided with neutral terminals, means for maintaining a balance between the currents flowing from said main transformer secondary windings comprising means for producing a direct current bias potential responsive to the average currents ilowing from one of said pair of secondary windings of said main power transformer, means for producing a direct current bias potential proportional to the average currents flowing from said other-secondary winding of said main power transformer, and means "order to restore balanced current conditions.

11. In combination an alternating current supply circuit, a direct current load circuit, an electric translating apparatus connected between said circuits and including a power transformer having a primary winding connected to said alternating current circuit, and a pair of secondary windings each associated with a plurality,of electric discharge valves for transmitting energy from said alternating current circuit to said direct current circuit, a grid transformer having a pair of secondary windings one of which is associated with the electric discharge valves connected to the other secondary winding of said main power transformer while the other is associated with the electric discharge valves connected to the other secondary winding of the main power transformer, means for balancing the currents flowing from said main transformer secondary windings comprising capacitance means responsive to the difl'erence between the average currents flowing from one of said main power secondarywindings and the average currents flowing from the other of said main power transformer secondary windings for impressing a direct current bias potential on the excitation circuits of the electric discharge valves as will advance the phase of the grid excitation of certain of said valves while retarding the phase of the grid excitation" of others of said valves.

12. In an electric valve converting apparatus, a supply circuit, a load circuit, electric translating apparatus connected between said circuits and including a plurality of inductive windings and a plurality of groups of electric discharge valves. each group of said valves being associated with one of said inductive windings, an excitation circuit for each of said groups or electric discharge valves. means for maintaining balanced conditions between the currents flowing in said inductive windings including a plurality oi! current transformers and associated rectiflers for producing unidirectional potentials proportional to the currents flowing in said inductive windin s. and means tor comparing said unidirectional potentials to obtain a resultant potential of such magnitude and polarity as to shiit the phase of the excitation of said groups of electrio discharge valves in such a manner as to restore balanced current conditions.

is; In an electric valve converting apparatus,

an alternating current supply clrcuit, a load cirsuit. eledtric translating apparatus connected between said circuits including a plurality of inductive windings and a plurality oi groups of electric discharge valves, each of said groups unbalance between the currents flowing from each 0! said inductive windings including ineans for producing direct current potentials responsive to the current flowing from each of saidinductive. windings, potentioineters for comparing said direct current potentials so as to obtain a resultant potential it any unbalance exists, and means for impressing such resultant potential upon the excitation circuit oi said electric discharge valves to restore balanced current conditions.

" ELMO E. MOYER.

LYSLE W. MORTON. AUGUST SCHMIDT, Ja.

v CERTIFICATE OF CORRECTION. o Patent No. 2,215, 71. May 27, 19in.

ELMO E. MOYER, ET AL.

It is hereby certified that error appears in the printed specification ofthe above mnnbered pateht requiring correction'as follows: Page}, second column, line 14 8, for "winding 71" read -windihg 21; page 5, first column, line 20, for gereter" read --greater-; page 5, first columh, line 62, claim 8, for "shart" read --share-; a d second column, 111N514 claim 11, for "the other secondary winding" read --one of said secondary windlugs"; and that the said Letters Patent should be read with thiscorrection therein that the same may confom to the record of the case in'the' Patent Office.

Signed and sealed this 15th day of July, A. D.- 19in.

Henry fan Aradale, 8 Acting Commissioner of Patents.

- CERTIFICATE OF CORRECTION; A 1 Patent No. 2,215, 71. May27, 19m. ELMO E. 'MOYER, ET AL.

It is hereby certified that error appears in the printed specification 4 of the above numbered patent requiring correction'ss follows: Page}, second column, line 14 8, for "winding 71" read -winding 21-; page 5, first column, line 20, for "gerater" read --greater--; page 5, first column, line 62, claim 8, for "shart" read -share--; and second column, line claim 11, for "the other secondary winding" read --one of said secondary windings-; and that the said Letters Patent should be read with this correction therein that the same may confozm to the record of the case in the Patent Office.

Signed and sealed this 15th day or July, A}. D; 191;;1.

Henry Ian Aradale, a Acting Commissioner of Patents. 

